Pressure-sensitive recording materials utilizing the color-forming reaction between a color former, i.e., a substance forming color on donating an electron or on accepting a proton, e.g., an acid, and a color developer, i.e., a substance accepting an electron or donating a proton, e.g., clay compounds such as acid clay, activated clay, attapulgite, zeolite, bentonite, and kaolin, aromatic carboxylic acid metal salts, and a phenol-formaldehyde resin, have heretofore been well known as described in, for example, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, Japanese Patent Publication (unexamined) Nos. 28411/74, 44009/75.
A color former layer of a pressure-sensitive copying paper is formed by dissolving a color former in a solvent, dispersing the solution thus prepared in a binder or encapsulating the solution, and then coating the dispersion or microcapsules as prepared above on a support such as paper, a plastic film, and a resin-coated paper.
A color developer sheet is prepared by dissolving or dispersing a binder in a medium, e.g., water, in combination with a binder, and then coating the solution or dispersion thus prepared on a support.
In general, the color former and the color developer are coated on the same surface or opposite surfaces of a support or on different supports. The color former in the microcapsule is released on application of pressure by writing or striking and comes into contact with the color developer, forming color and producing a recorded image or pattern.
As described above, various compounds are known as color developers for use in pressure-sensitive recording materials. Of these compounds, clay compounds such as activated clay and acid clay are presently in widespread use as the color developer. These clay compounds, however, have disadvantages in that the light resistance and water resistance of the final color forming member are poor and further the storage stability is poor.
It is also known that aromatic carboxylic acid salts are color developers free from the disadvantages as described above.
A color developer sheet containing such an aromatic carboxylic acid metal salt as a color developer is ordinarily prepared by mechanically dispersing a mixture of the color developer and an inorganic pigment, a binder, a dispersant and other additives in water, and then coating the thus prepared coating solution on a support.
Japanese Patent Publication (unexamined) No. 143322/79 discloses a method of preparing a color developer sheet which comprises dissolving the aromatic carboxylic acid metal salt color developer in an organic solvent, emulsifying the resulting solution in water to prepare a coating solution, and then coating the coating solution on a support. A pressure-sensitive copying sheet including a color developer sheet prepared by the method as described above has advantages in that the color developer reacts with the color former with high efficiency and exhibits excellent color-forming properties even if it is added in a small amount, and moreover, the color-forming rate is very high. However, it has been found that the pressure-sensitive copying sheet has serious disadvantages in that the light resistance of the color forming member is seriously reduced, and when the sheet is used as an intermediate sheet, printing fog is markedly formed. The intermediate sheet as described above means a material for multi-sheet copying in which a color former layer is provided on one side and a color developer layer is provided on the opposite side. The term "printing fog" as used above means the phenomenon that when the color former layer or color developer layer is printed, the microcapsules are broken by the printing pressure and the color developer released from the microcapsules permeates through the paper and comes into contact with the color developer coated on the opposite surface, forming color.
It has, therefore, been desired to overcome the problems as described above.